High-horsepower electric motors were utilized to drive large compressors (made of 4340 steel shafts and gear-type couplings) required in a manufacturing process. The load was transmitted by two keys 180 deg apart. Six of the eight compressor shafts were found cracked in a keyway and one of them fractured after a few months of operation. Visual examination of fractured shaft revealed that the cracks originated from one of the keyways and propagated circumferentially around the shaft. The shaft and coupling slippage was indicated by the upset keys and this type of fracture. The shaft surface both near and in the keyways indicated fretting which greatly reduced the fatigue limit of the shaft metal and initiated fatigue cracks. Fatigue marks were observed on the fractured key. Repetitive impact loading was responsible for propagation of the cracks. The high cyclic bending stresses were caused by misalignment between the electric motor and compressor and were transmitted to the shaft through the geared coupling. Flexible-disk couplings capable of transmitting the required horsepower were installed on the shafts as a corrective measure.

Overhaul mechanics discovered a crack in an AISI 4340 Cr-Mo-Ni alloy steel pivot bolt when grinding off the chromium plating. The bolt had served for an estimated 10,000 h and was replated when last overhauled. On checking the bolt, several fine cracks were found on the surface. A 6500x micrograph revealed the intergranular nature of a crack. By trying different grinding procedures, investigators were able to reproduce this type of failure in the laboratory. It was concluded that grinding cracks initiated the failure. It should be noted that governing specifications prohibit grinding on high-strength steel; chromium should be stripped by electrochemical methods.

An AISI 4340 threaded steel connecting rod that was part of a connecting linkage used between a parachute and an instrumented drop test assembly fractured under high dynamic loading when the assembly was dropped from an airplane. A large flaw that originated from the root of a machined thread groove was visible on the fracture surface. Heavy oxidation at elevated temperatures was indicated as most of the surface of the flaw was black. Fine secondary cracks aligned transverse to the growth direction was revealed by scanning electron microscopy. It was established that intergranular cracking observed in this alloy was caused during heat treating as the thread root served as an effective stress concentration and induced quench cracking. It was found that fracture in the overload region occurred by a ductile void growth and coalescence process. Premature failure of the threaded rod was thus attributed to the presence of the quench crack flaw caused by an improper machining sequence and heat treatment practice.

The draw-in bolt and collet from a vertical-spindle milling machine broke during routine cutting of blind recesses after a relatively long service life. The collet ejected at a high rotational speed due to loss of its vertical support and shattered one of its arms upon impact with the work table. SEM fractography and metallographic examinations conducted on the bolt revealed hairline indications along grain facets on the fracture surface and stepwise cracking in the material, both indicating failure by hydrogen embrittlement. Similar draw-in bolts were discarded and replaced with bolts manufactured using controlled processes.

Military aircraft use a cartridge ignition system for emergency engine starts. Analysis of premature failures of steel (AISI 4340) breech chambers in which the solid propellant cartridges were burned identified corrosion as one problem with an indication that stress-corrosion cracking may have occurred. A study was made for stress-corrosion cracking susceptibility of 4340 steel in a paste made of the residues collected from used breech chambers. The constant extension rate test (CERT) technique was employed and SCC susceptibility was demonstrated. The residues, which contained both combustion products from the cartridges and corrosion products from the chamber, were analyzed using elemental analysis and x-ray diffraction techniques. Electrochemical polarization techniques were also utilized to estimate corrosion rates.

Examination of a cracked nose landing gear cylinder made of AISI 4340 Cr-Mo-Ni alloy steel proved that the part started to fail on the inside diam. When the nucleus of the stress-corrosion crack was studied in detail, iron oxide was found on the fracture surface. A 6500x micrograph revealed this area also displayed an intergranular texture. One of a group of small grinding cracks on the ID of the cylinder nucleated the failure. Other evidence indicated the cracks developed when the cylinder was ground during overhaul.

Ground maintenance personnel discovered hydraulic fluid leaking from two small cracks in a main landing gear cylinder made from AISI 4340 Cr-Mo-Ni alloy steel. Failure of the part had initiated on the ID of the cylinder. Numerous cracks were found under the chromium plate. A 6500x electron fractograph showed cracking was predominantly intergranular with hairline indications. Leaking had occurred only 43 h after overhaul of the part. Total service time on the part was 9488 h. It was concluded that cracking on the ID was caused by hydrogen embrittlement which occurred during or after overhaul. The specific source of hydrogen which produced failure was not ascertainable.

A nose landing gear cylinder made from AISI 4340 Ni-Cr-Mo alloy steel was found cracked and leaking, causing partial depressurization. Investigation revealed the crack to be a stress-corrosion type, judging by the 6500x electron fractograph. It had started in a region of concentrated, large non-metallic inclusions near the chromium-plated ID of the cylinder. Also, there were breaks in the chromium plate and pits in the underlying base metal. The cylinder had been in service for 18,017 h, and 5948 h had passed since the first and only overhaul. Substandard plating of the ID at this time ultimately resulted in pitting of the metal. The combination of surface pitting and stress concentration at the nearby inclusions resulted in stress-corrosion cracking.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

A cadmium-plated 4340 Ni-Cr-Mo steel ballast elbow assembly was submitted for failure analysis to determine the element or radical present in an oxidation product found inside the elbow assembly. Energy-dispersive x-ray analysis in the SEM showed that iron was the predominant species, presumably in an oxide form. The inside surface had the appearance of typical corrosion products. Hardness measurements indicated that the 4340 steel was heat treated to a strength of approximately 862 MPa (125 ksi). It was concluded that the oxide detected on the ballast elbow was iron oxide. The possibility that the corrosion products would eventually create a blockage of the affected hole was great considering the small hole diameter (4.2 mm, or 0.165 in.). It was recommended that a quick fix to stop the corrosion would be to apply a corrosion inhibitor inside the hole. This, however, would cause the possibility of inhibitor buildup and the eventual clogging of the hole. A change in the manufacturing process to include a cadmium plating on the hole inside surface was recommended. This was to be accomplished in accordance with MIL specification QQ-P-416, Type II, Class 1. A material change to 300-series stainless steel was also recommended.

A splined shaft on a wood chip-to-fiber refiner failed during equipment start-up. The shaft broke into two pieces at a location close to the end of the splined part of the shaft. The failed component showed the classical fatigue-cracking fracture face. The shaft had a diam of approximately 140 mm (5.5 in.) in the unsplined section and was made of 4340 Ni-Cr-Mo alloy steel heat treated to a uniform hardness of HRC 31. Cracks from at least seven different origins had coalesced to produce the single large crack that resulted in failure. The origins of these cracks were on the flanks of the splines. SEM examination revealed the splined shaft failed by fretting fatigue.

Several large chromium-plated 4340 steel cylinders were removed from service because of deep longitudinal score marks in the plating. One of the damaged cylinders and a mating cast aluminum alloy B850-T5 bearing adapter that also exhibited deep longitudinal score marks were submitted for examination. Analysis (visual inspection, manual testing of the hardness and adherence of the chromium plating, 100x microscopic examination, and hardness testing) supported the conclusions that high localized loads on the cylinder had resulted in chipping of the chromium plating, particles of which became embedded in the aluminum alloy adapter. The sliding action of the adapter with embedded hard particles resulted in scoring of both the cylinder and the adapter. If the cylinder alone had been available for examination, it might have been concluded that the scoring had been caused by entrapped sand or debris from an external source. No recommendations were made.

The 4340 steel main rotor yoke of a helicopter failed during a hovering exercise. Visual examination of the yoke revealed no evidence of gross external damage. Visual fracture surface examination, macrofractography, scanning electron micrography, and metallography of a section cut from the yoke in the region of the cracking indicated that the failure was caused by fatigue-crack initiation and growth from severe corrosion damage to a pillow-block bolt hole. Corrosion occurred because of failure of the protection scheme. An upgraded corrosion protection scheme for the bolt holes was recommended, along with nondestructive inspection of the region at intervals determined by fractographic analysis of the fatigue crack growth.

Two vertical coal-pulverizer shafts at a coal-fired generation station failed after four to five years in service. One shaft was completely broken, and the other was unbroken but cracked at both ends. shaft material was AISI type 4340 Ni-Cr- Mo alloy steel, with a uniform hardness of approximately HRC 27. Metallographic examination of transverse sections through the surface-damaged areas adjacent to the cracks also showed additional small cracks growing at an angle of approximately 60 deg to the surface. The crack propagation mode appeared to be wholly transgranular. SEM examination revealed finely spaced striations on the crack surfaces, supporting a diagnosis of fatigue cracking. Crack initiation in the pulverizer shafts started as a result of fretting fatigue. Greater attention to lubrication was suggested, combined with asking the manufacturer to consider nitriding the splined shaft. It was suggested that the surfaces be securely clamped together and that an in-service maintenance program be initiated to ensure that the tightness of the clamping bolts was verified regularly.

A steering knuckle used on an earthmover failed in service. The component fractured into a flange portion and a shaft portion. The flange was 27.9 cm (11 in.) in diam around which there were 12 evenly spaced 16 mm diam bolt holes. The shaft was hollow with a 10.5 cm (4 in.) OD and a wall thickness of 17 mm. The steering knuckle was made of 4340 steel and heat treated to a hardness of about 415 HRB (yield strength of about 1069 MPa, or 155 ksi). The vehicle had been involved in a field accident six months before the steering knuckle failed. Several components, including portions of the frame, had been damaged and replaced, but there was no observed damage to the steering. Analysis supported the conclusion that the fracture was the result of the prior accident, the most likely explanation being that the shaft was bent and that continued use caused a crack to initiate and propagate to fracture. No evidence of a defective design, improper microstructure, high inclusion count, or other stress-raising condition was observed. No recommendations were made.

Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, planar dimpled facets along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.

One of the two AISI 4340 steel pitch horn bolts from the main rotor hub assembly failed while in service. Optical microscope revealed evidence of corrosion pitting in regions adjacent to the fracture. Fractographic examination utilizing a scanning electron microscope revealed multiple crack origins which assumed a “thumbnail” shape and displayed surface morphologies which resulted from intergranular decohesion. Many of the crack sites initiated from corrosion pits. Energy dispersive spectroscope performed on areas within the crack initiation site showed the presence of chlorides. The failure was attributed to stress-corrosion cracking. Short- and long-term recommendations to prevent future failures are given.

A forged, cadmium-plated electroslag remelt (ESR) 4340 steel mixer pivot support of the rotor support assembly located on an Army attack helicopter was found to be broken in two pieces during an inspection. Visual inspection of the failed part revealed significant wear on surfaces that contacted the bushing and areas at the machined radius where the cadmium coating had been damaged, which allowed corrosion pitting to occur. Optical microscopy showed that the crack origin was located at the machined radius within a region that was severely pitted. Electron microscopy revealed that most of the fracture surface failed in an intergranular fashion. Energy dispersive spectroscopy determined that deposits of sand, corrosion and salts were found within the pits. The failure started by hydrogen charging as a result of corrosion, and was aggravated by the stress concentration effects of pitting at the radius and the high notch sensitivity of the material. The failure mechanism was hydrogen-assisted and was most likely a combination of stress-corrosion cracking and corrosion fatigue. Recommendations were to improve the inspection criteria of the component in service and the material used in fabrication.

A 4340 steel piston engine crankshaft in a transport aircraft failed catastrophically during flight. The fracture occurred in the pin radius zone. Fractographic studies established the mode of failure as fatigue under a complex combination of bending and torsional stresses. SEM examination revealed that the fracture origin was a subsurface defect-a hard refractory (Al2O3) inclusion—in the zone close to the pin radius. Chemical analysis showed the crankshaft material to be of inferior quality. It was recommended that magnetic particle inspection using the dc method be used to cheek for cracks during periodic maintenance overhauls.

A hollow, splined alloy steel aircraft shaft (machined from an AMS 6415 steel forging – approximately the same composition as 4340 steel – then quenched and tempered to a hardness of 44.5 to 49 HRC) cracked in service after more than 10,000 h of flight time. The inner surface of the hollow shaft was exposed to hydraulic oil at temperatures of 0 to 80 deg C (30 to 180 deg F). Analysis (visual inspection, 15-30x low magnification examination, 4x light fractograph, chemical analysis, hardness testing) supported the conclusions that the shaft cracked in a region subjected to severe static radial, cyclic torsional, and cyclic bending loads. Cracking originated at corrosion pits on the smoothly finished surface and propagated as multiple small corrosion-fatigue cracks from separate nuclei. The originally noncorrosive environment (hydraulic oil) became corrosive in service because of the introduction of water into the oil. Recommendations included taking additional precautions in operation and maintenance to prevent the use of oil containing any water through filling spouts or air vents. Also, polishing to remove pitting corrosion (but staying within specified dimensional tolerances) was recommended as a standard maintenance procedure for shafts with long service lives.